Biomaterials are synthetic materials used to replace or augment a part of a living system or to function in contact with living tissue. Among the many causes for mortality among human beings, cardiovascular diseases account for a major portion of such deaths. Therefore, continuous improvements in the development of new and improved biomaterials capable of replacing parts of the cardiovascular system is extremely important. The primary requirements for biomaterials for long-term implants, e.g. heart valve prostheses, stents, and vascular grafts, are biocompatibility, thrombresistivity, nontoxicity, and durability. Furthermore, biomaterials should be nonirritating to tissue and nondegradable in the harsh physiological environment, neither absorbing blood constituents nor releasing foreign substance into the bloodstream.
A key problem in interfacing a biomaterial with blood revolves around the characteristics of the implant surface. Thrombus and embolism formation at the blood-implant interface is of foremost concern, and is the technology limiting factor in implant design and materials selection.
Metal implants have been used as implants due to their toughness, i.e., the ability to absorb energy before fracture. The metals and metallic alloys commonly used for implants form passivating oxide surface layers at their interface with air. However, the passivation may become destabilized by the acidic (e.g., pH of about 5), saline nature of blood. The surface reactivity of the metallic implants at the blood-implant interface leads to bulk electrochemical corrosion and localized stress corrosion, which in turn, leads to mechanical failure of the implant and ion contamination of the blood.
Polymer implants or polymer-coated metal implants are commonly used for cardiovascular applications because of their good initial biocompatibility. However, the use of polymers poses a major problem, namely, chemical degradation over time resulting in thrombus and embolism formation as well as the production of harmful wear-related debris.
Ceramic implants offer a compromise by providing chemical inertness, hardness, and wear-resistance. However, ceramic implants exhibit the same major drawback as that for all traditional bulk ceramic structures in their inability to deform plastically under either or both static and cyclic loading. The lack of flexibility of the bulk ceramics leads to difficulty in manufacturing implants and microcracking during the implant's lifetime in the body due to fatigue failure.